Effect of CO32- incorporation on the cproperties of wet chemically synthesized β-tricalcium phosphate (TCP) ceramics

Calcium-deficient hydroxyapatite (CDHA) powders were synthesized by a wet chemical precipitation method using Ca(OH)(2) and H3PO4 solutions. Single-phase beta-tricalcium phosphate (beta-TCP) powder with a molar (Ca+Mg)/P ratio of 1.5 was obtained after calcination of CDHA synthesized under vacuum. During synthesis in air, CO2 can be adsorbed and HBO42- is partly substituted by CO32-, resulting in a lower phosphorous content and consequently an increase of the molar (Ca+Mg)/P ratio to 1.53. A two-phase beta-TCP powder containing 20 wt% hydroxyapatite (HA) was obtained after calcination. Samples prepared from beta-TCP powders synthesized under vacuum achieved a compressive strength of 301 +/- 23 MPa at 99.6% fractional density, while TCP/HA samples prepared in air achieved a maximum compressive strength of 132 +/- 29 MPa at 91.7% fractional density. This decrease in strength can be correlated to the porosity retaining due to CO2 release during sintering and residual tensile stresses in the TCP matrix caused by the thermal expansion mismatch of beta-TCP and HA.